The Pentagon's press service said Panetta will take part in a two-day NATO meeting on the end strategy for the war in Afghanistan. NATO Secretary-General Anders Fogh Rasmussen is to meet Thursday with NATO-member defense ministers.
U.S. President Barack Obama, in his State of the Union address, said 34,000 U.S. troops stationed in Afghanistan will be back home by this time next year. Afghan forces are taking the lead in some combat operations as international forces prepare to pull out of the country next year.
The United Nations this week reported that civilian deaths as a result of conflict in Afghanistan for the first time in six years. Attacks against civilians, however, are on the rise.
Panetta is retiring following his trip to Brussels. Obama has nominated former U.S. Sen. Chuck Hagel, R-Neb., as his replacement.
Hagel faced a tough nomination hearing as Republicans questioned his stance on foreign policy issues ranging from Iraq war strategy to Israeli alliance.
Sen. John McCain, R-Ariz., a member of the Senate Armed Services Committee, said during on NBC's "Meet the Press" that "Hagel will probably have the votes necessary to be confirmed" when lawmakers return from recess.